The truth about soy

Jane Murphy / 15 February 2017 ( 06 January 2021 )

Soy has been hailed as a superfood, but some studies suggest it may not be such a healthy choice. So what's the truth behind the headlines?



What exactly is soy?

Soy – or soya – is a bean with many guises. It can be eaten fresh from the pod (known as an edamame bean); unfermented as soya milk and other dairy alternatives, tofu (bean curd) and meat replacements, such as mince and textured vegetable protein (TVP); and fermented as tempeh (bean cake) and miso (bean paste).

How to incorporate tofu into your diet

What are soy's main health benefits?

'It's an excellent source of protein, so can be especially beneficial for vegetarians and vegans, who don't eat any animal protein,' says Rob Hobson, registered nutritionist at Healthspan and co-author of The Detox Kitchen Bible. 'It also contains calcium, which is needed for good bone health, so may be of particular benefit for post-menopausal women, who can lose up to 10 per cent of their bone density.'

But calcium isn't the only bone-building hero found in soy: it's also the main dietary source of plant chemicals called isoflavones, which are similar in structure to the human hormone oestroge, which dwindles at menopause. Indeed, when researchers at the University of Hull gave a group of women in early menopause a daily supplement containing soy proteins and isoflavones they found clues that it helped slow the rate of bone loss, according to a study published in 2017.

Soy is also a good source of essential minerals including iron, magnesium and potassium, as well as vitamins B6, E, K and folate. It's naturally low in saturated fats and sodium, and has a low glycaemic index, meaning the carbohydrates it contains are released slowly and steadily to help keep blood sugar levels steady.

Do you want to eat less meat? Try these protein alternatives

Great! Anything else in soy’s favour?

Some research suggest that soy isoflavones can help reduce menopausal hot flushes and other post-menopausal problems. However, a 2015 British Pharmacological Society study review concluded that these positive effects were generally 'slight and slow' compared to the oestrogen found in HRT.

That said a 2019 systematic review published in the journal Nutrients concluded that, although it’s hard to be definitive due to inconsistencies in how studies are carried out, soy isoflavones reduce hot flushes and can help ease other post-menopausal problems. Benefits include helping reduce loss of bone density as well as lowering systolic blood pressure during early menopause. Meanwhile, in the test tube at least, they improve control of blood glucose. Say the researchers, “The safety profile of isoflavones combined with their benefit to overall health makes them a compelling treatment option for postmenopausal women unwilling or unable to take hormone replacement therapy.”

Heart health benefits of soy

There’s growing evidence too that soy can help lower levels of ‘bad’ LDL cholesterol and support heart health. A study of the eating habits of 200,000 people published in March 2020 in the American Heart Association journal Circulation, for example, found that people who ate at least one serving of tofu a week had an 18% lower risk of heart disease compared to those who rarely ate tofu. In fact many heart health organisations including Heart UK, which suggests putting soy on the menu as part of its Ultimate cholesterol lowering plan, now recommend soy as part of a heart healthy diet.

So what are the cons of soy?

'Much of the controversy surrounding soy is related to breast cancer risk and treatment,' Hobson explains. 'But the consensus from the available research is that consuming moderate amounts of soy – one to three servings daily – as part of a balanced diet is fine.'

The American Institute for Cancer Research observes that there is no higher risk for breast cancer survivors who consume soy foods, according to studies. In fact, they note, emerging evidence suggests ‘potential for greater overall survival and perhaps decreased recurrence, among women a year or more after diagnosis who include moderate amounts of soy’ in their diet. Good news indeed!

Can soy affect the thyroid?

Soy isoflavones have also been linked to low thyroid function. However a 2019 systematic review and meta-analysis scotches this suggestion. It concluded that s oy supplementation has no effect on thyroid hormones and only very modestly raises levels of the master thyroid hormone in the brain, TSH, the clinical significance, of which – if any - say the researchers isn’t clear. So again the pendulum has swung in favour of soy.

Some of the other plant chemicals found in soy, such as phytates and oxalate, can sometimes inhibit the absorption of minerals, including calcium, iron and zinc. However, recent studies show these minerals are usually well absorbed from soya when eaten as part of a healthy, balanced diet. What's more, the phytate content can be significantly reduced during the fermentation process – so it's not always a concern.

How does soya milk compare to cow's milk?

'Always choose a soya milk that's been fortified with vitamins and minerals, such as bone-building vitamin D,' Hobson advises.'Other than that, soya milk contains around the same amount of calories, but less fat and protein than semi-skimmed milk. Cow's milk contains more vitamin B12, which is essential for brain health, than soya milk and around the same amount of calcium.'

Some other plant chemicals found in soy, such as phytates and oxalate, may inhibit the absorption of minerals, including calcium, iron and zinc. However, recent studies show these are usually well absorbed from soy when eaten as part of a healthy, balanced diet. What's more, the phytate content can be significantly reduced during fermentation – so it's not always a concern.

'Always choose a soya milk fortified with vitamins and minerals, such as bone-building vitamin D,' Hobson advises. 'Other than that, soya milk contains around the same amount of calories, but less fat and protein than semi-skimmed milk. Cow's milk contains more vitamin B12, which is essential for brain health, than soya milk and around the same amount of calcium.'

Is the vegan diet healthy?

So it's OK to keep soy on my shopping list then?

'Yes,' says Hobson. 'But do opt for soy foods that aren't genetically modified or overly processed. Fermented varieties, such as miso and tempeh, appear to be most beneficial, but all can still offer a healthy contribution to your diet.'

Cooking with soy

Visit our tofu recipes section for more cooking ideas

Unlimited access to a qualified GP with Saga Health Insurance - you'll have access 24 hours a day, 365 days a year to a GP consultation service. Find out more about our GP phone service.


The opinions expressed are those of the author and are not held by Saga unless specifically stated.

The material is for general information only and does not constitute investment, tax, legal, medical or other form of advice. You should not rely on this information to make (or refrain from making) any decisions. Always obtain independent, professional advice for your own particular situation.